1. Field of the Invention
The present invention relates to a protective means for a series capacitor in a high-voltage network, said means comprising a voltage-dependent resistor built up of metal oxide varistors and arranged in parallel with the series capacitor, a spark gap arranged in parallel with the voltage-dependent resistor for shunting the resistor in case of overload thereof, and a trigger device for firing the spark gap.
2. Prior art
For protection of a series capacitor it is known to arrange, in parallel with the capacitor, a voltage-dependent resistor means built up of metal oxide varistors in the form of ceramic blocks. If such a resistor means is to be dimensioned for the entire short-circuit power of the network, it will become large and expensive. Because of the high energy absorption capacity required, the means, in addition, has to be provided with several parallel resistor branches, which requires special measures in order to achieve a controlled current division between different parallel branches (see, e.g., U.S. Pat. No. 4,347,539). Another serious drawback in a protective means having only parallel resistors is that if, for example due to an overload, a fault with a resulting short-circuit occurs in one or more of the resistor blocks, an internal overpressure may be generated in the porcelain housing of the resistor, whereby the housing may crack resulting in a total breakdown.
It is also known, in protective means of the above-mentioned kind, to arrange a protective spark gap, provided with forced triggering, in parallel with the voltage-dependent resistor. The cost of the resistor can therefore be kept low by dimensioning the resistor to absorb only a limited amount of energy. When the absorbed energy exceeds the level predetermined by the dimensioning, the parallel spark gap is activated and is then extinguished by external current interruption or shunting. Different trigger devices for a protective spark gap of said kind are known. In these devices, the energy for triggering the spark gap is taken from a current transformer arranged in series with the resistor (see, e.g., U.S. Pat. Nos. 4,174,529 and 4,432,029). Since the triggering may require relatively high power, this transformer will have a high rating and therefore becomes expensive. Further, these devices are provided with switching devices (e.g. in the form of transsistor switches) which are complicated and expensive.